I'm working on a side-project in my spare time since the end of 2012 (before at the university) with some gaps, which is an append-only DBS with time travel capabilities using a custom storage engine based on COW tries and a (still I think) novel page versioning strategy called Sliding Snapshot. Recently I began work on pooling pages for reuse and a custom allocator for variable page sizes. Before, I had created new instances whenever a page was read from disk instead of reusing instances, so the allocation rate was really high for parallel transactions.
https://sirix.io | https://github.com/sirixdb/sirix | http://brackit.io